1. Field of the Invention
The present invention relates to the equipment and methods in oil field operations. Particularly, the invention relates to helical gear pumps.
2. Background of the Related Art
Helical gear pumps, typically known as progressive cavity pumps/motors (herein PCPs), are frequently used in oil field applications, for pumping fluids or driving downhole equipment in the wellbore. A typical PCP is designed according to the basics of a gear mechanism patented by Moineau in U.S. Pat. No. 1,892,217, incorporated by reference herein, and is generically known as a xe2x80x9cMoineauxe2x80x9d pump or motor. The mechanism has two helical gear members, where typically an inner gear member rotates within a stationary outer gear member. In some mechanisms, the outer gear member rotates while the inner gear member is stationary and in other mechanisms, the gear members counter rotate relative to each other. Typically, the outer gear member has one helical thread more than the inner gear member. The gear mechanism can operate as a pump for pumping fluids or as a motor through which fluids flow to rotate an inner gear so that torsional forces are produced on an output shaft. Therefore, the terms xe2x80x9cpumpxe2x80x9d and xe2x80x9cmotorxe2x80x9d will be used interchangeably herein.
FIG. 1 is a schematic cross sectional view of a pumping/power section of a PCP. FIG. 2 is a schematic cross sectional view of the PCP shown in FIG. 1. Similar elements are similarly numbered and the figures will be described in conjunction with each other. The pumping section 1 includes an outer stator 2 formed about an inner rotor 4. The stator 2 typically includes an outer shell 2a and an elastomeric member 10 formed therein. The rotor 4 includes a plurality of gear teeth 6 formed in a helical thread pattern around the circumference of the rotor. The stator 2 includes a plurality of gear teeth 8 for receiving the rotor gear teeth 6 and typically includes one more tooth for the stator than the number of gear teeth in the rotor. The rotor gear teeth 6 are produced with matching profiles and a similar helical thread pitch compared to the stator gear teeth 8 in the stator. Thus, the rotor 4 can be matched to and inserted within the stator 2. The rotor typically can have from one to nine teeth, although other numbers of teeth can be made.
Each rotor tooth forms a cavity with a corresponding portion of the stator tooth as the rotor rotates. The number of cavities, also known as stages, determines the amount of pressure that can be produced by the PCP. Typically, reduced or no clearance is allowed between the stator and rotor to reduce leakage and loss in pump efficiency and therefore the stator 2 typically includes the elastomeric member 10 in which the helical gear teeth 8 are formed. Alternatively, the elastomeric member 10 can be coupled to the rotor 4 and engage teeth formed on the stator 2 in similar fashion. The rotor 4 flexibly engages the elastomeric member 10 as the rotor turns within the stator 2 to effect a seal therebetween. The amount of flexible engagement is referred to as a compressive or interference fit.
FIG. 3 is a cross sectional schematic view of diameters of the stator shown in FIGS. 1 and 2. A typical stator 2 has a constant minor diameter 3a defined by a circle circumscribing an inner periphery of the stator teeth 8. The typical stator also has a constant major diameter 5a defined by a circle circumscribing an outer periphery of the teeth 8. A thread height 7a is the height of the teeth, which is the difference between the major diameter and the minor diameter divided by two, i.e., a minor radius subtracted from a major radius.
FIG. 4 is a cross sectional schematic view of diameters of the rotor shown in FIGS. 1 and 2. The rotor 4 has minor and major diameters and a thread height to correspond with the stator. The typical rotor has a minor diameter 3b defined by a circle circumscribing an inner periphery of the teeth 6. The rotor also has a major diameter Sb defined by a circle circumscribing an outer periphery of the teeth 6. The thread height 7b is the difference between the major diameter and the minor diameter divided by two.
A PCP used as a pump typically includes an input shaft 18 that is rotated at a remote location, such as a surface of a wellbore (not shown). The input shaft 18 is coupled to the rotor 4 and causes the rotor 4 to rotate within the stator 2, as well as precess around the circumference of the stator. Thus, at least one progressive cavity 16 is created that progresses along the length of the stator as the rotor is rotated therein. Fluid contained in the wellbore enters a first opening 12, progresses through the cavities, out a second opening 14 and is pumped through a conduit coupled to the PCP. Similarly, a PCP used as a motor allows fluid to flow from typically a tubing coupled to the PCP, such as coiled tubing, through the second opening 14, and into the PCP to create hydraulic pressure. The progressive cavity 16 created by the rotation moves the fluid toward the first opening 12 and is exhausted therethrough. The hydraulic pressure, causing the rotor 4 to rotate within the stator 2, provides output torque to an output shaft 19 used to rotate various tools attached to the motor.
The rubbing of the rotor in the stator as the rotor rotates causes several problems. Various operating conditions change the interference fit and therefore a predetermined amount of interference is difficult at best to obtain for efficient performance under the varying conditions. For example, the rubbing causes the elastomeric member to wear. The amount of interference is reduced and, therefore, the amount of pressure or output torque that the PCP can produce is also reduced. Further, the interference fit between the rotor and stator is especially prone to deterioration from particulates in a production fluid. Still further, the rubbing itself produces heat buildup in the elastomeric member and decreases the life of the elastomeric member. As another example, a PCP can encounter fluctuations in operating temperatures. For example, some wellbore operations inject steam downhole through the pump into a production zone and then reverse the flow to pump production fluids produced by the wellbore at a different temperature up the wellbore. The temperature fluctuations can cause the components, particularly the elastomeric member, to swell and change the interference fit between the stator and rotor. The swelling creates additional loads on the pump and to a corresponding input device, such as an electric motor used to rotate the shaft 18 and the rotor 4 of the PCP. Further, swelling can occur with time of use and with chemicals existing in production fluids. The swelling can be great enough to damage the pump and require repair or replacement.
Some proposed solutions by those in the art include preloading the elastomeric member, so that the elastomeric member compensates to maintain a given interference fit as wear occurs. Others have proposed an inflatable bladder type of elastomeric member than can be expanded to increase the interference fit. One solution offered by U.S. Pat. No. 5,722,820 seeks to equalize pressures across the several stages of the PCP, and thereby reduce the heat buildup. The amount of interference fit is gradually reduced in subsequent stages by gradually reducing either the rotor diameter or increasing the stator diameter. However, the reference does not address adjustments needed to solve the problems of swelling or deterioration or the varying operating conditions.
Therefore, there exists a need for providing a PCP that can be adjusted to a variety of selected interference fits or even clearances to meet various operating conditions.
The present invention provides an adjustable rotor and/or stator, so that the interference fit and/or clearance can be adjusted. The rotor and/or stator are tapered to provide a difference in fit between the rotor and stator by manual or automatic longitudinal adjustment of their relative position. In one embodiment, the adjustment may occur while the PCP in mounted downhole in a wellbore. In another embodiment, the adjustment may occur automatically depending on sensor input of operating conditions of the PCP.
In one aspect, a progressive cavity pump (PCP) is provided, comprising a stator having a helical internal bore with at least two helical threads, the stator being tapered at least partially between the inlet and the outlet, a rotor having a helical periphery with one helical thread less than the stator and disposed at least partially within the stator to form a plurality of cavities between the rotor and the stator, the rotor being tapered at least partially between the inlet and the outlet.
In another aspect, a method of adjusting a progressive cavity pump is provided, comprising inserting a progressive cavity pump into a wellbore, the pump comprising a stator having a helical internal bore with at least two helical threads, the stator being tapered at least partially between the inlet and the outlet, a rotor having a helical periphery with one helical thread less than the stator and disposed at least partially within the stator to form a plurality of cavities between the rotor and the stator, the rotor being tapered at least partially between the inlet and the outlet, longitudinally positioning the rotor relative to the stator at a first longitudinal position and adjusting the rotor relative to the stator to a second longitudinal position.
In another aspect, a progressive cavity pump having a inlet and an outlet is provided, comprising a stator having a helical internal bore with at least two helical threads, the stator having a first helical pitch, a rotor having a helical periphery with one helical thread less than the stator and disposed at least partially within the stator to form a plurality of cavities between the rotor and the stator, the rotor having a second helical pitch different from the first helical pitch at least partially between the inlet and the outlet.